You have to be very lucky to be one of the chosen few who receive an eBay coupon by email. These coupons are just like cash that you can use towards anything you buy on eBay, the only conditions being that you pay using PayPal, and that you are using eBay in the USA, Canada or the UK.
Of course, if you just wait, you'll be waiting for a long time. You have to do something to make yourself look like the kind of person eBay would want to tempt back with a coupon. If you open an account, buy a few medium-value things and then suddenly stop, the chances are you'll find yourself with some kind of special offer, but still, not always.